Holiday shoppers are expected to spend about 3.9% more this year than they did last year, according to a National Retail Federation estimate. And in order to facilitate the shopping experience, retailers are pulling out all the stops.

Black Friday, the day after Thanksgiving, has traditionally represented the start of the holiday shopping period. Over the past few years, a kind of arms race has developed among retailers, and stores have opened their doors earlier and earlier each Friday. In fact, in an effort to combat a shopping season that is nearly a full week shorter than last year.

It is possible that the earlier opening hours will not have the desired effect. Instead of increasing sales, they could just be pushing the spike in sales forward. Last year, many companies began selling on Thanksgiving for the first time. This reduced sales on Black Friday, and overall holiday spending had its smallest increase since 2009.

Holiday retail sales a year ago were up about 3.5% from the year before, despite the longer shopping hours. In 2011, they were up 5.6% over 2010. These figures certainly make it harder to argue that more Thanksgiving hours provide any boost in overall holiday sales.

Here is our rundown of opening hours at eight top retailers for the Thanksgiving-Black Friday weekend

Wal-Mart Stores posted more than $328 billion in retail sales at its more than 4,500 U.S. stores last year. Overall, Walmart's sales were greater than the seven other retailers on this list combined. But with Thanksgiving falling on November 28, later than in most years, and consumer budgets expected to be tight, the retail giant moved its opening hours earlier in an attempt to capture sales. Walmart also pushed up the start of its online annual holiday deals by one month, to early November.

2. Target

Thanksgiving day opening: 8:00 p.m.

Target plans to open stores at 8 p.m. on Thanksgiving, an hour earlier than last year. The stores will remain open all night and close on Black Friday at 11 p.m. However, Target had limited success on Black Friday last year. According to Placed, a location analytics firm, Target failed to match the performance of other top retailers on this critical shopping day. This year, the chain also plans to offer Thanksgiving deals to online shoppers and lead up to the Thanksgiving opening hour with a special online-only promotion of 15 daily discounts. The online promotion will last two weeks. In 2012, Target's 1,778 stores had revenues of nearly $72 billion, more than 5% more than the prior year.

3. Best Buy

Thanksgiving day opening: 6:00 p.m.

Best Buy announced last week that more than 1,000 stores will be open all night on Thanksgiving, starting at 6 p.m., and close on Friday at 10 p.m. Best Buy's U.S. sales totaled $34.4 billion in 2012, an increase of just 1.1% from the prior year. In recent years, the consumer electronics retailer has been has lost sales to online retailers. While traffic at the stores may have remained the same, many customers simply used the stores to test products before buying them at Amazon.com or other e-commerce sites, a phenomenon called "showrooming." This year the company is embracing its showroom reputation and hopes to boost sales by bringing in additional foot traffic. Best Buy also said that it would lead up to Thanksgiving with special offers for rewards card members in the days before the holiday.

Sears Holdings will open its Sears stores at 8 p.m. on Thanksgiving. The stores will remain open until 10 p.m. on Black Friday. At its Kmart stores, the shopping season will get an even earlier start. Kmart stores will open at 6 a.m. on Thanksgiving morning and remain open for 41 hours, until 11 p.m. Friday night. The company may be growing desperate for a winning holiday strategy after years of poor financial results. The company's U.S. retail sales declined by 7.5% in 2012 from the prior year to nearly $31 billion, and the retailer must try everything to boost sales this year.

5. Macy's

Thanksgiving day opening: 8:00 p.m.

Macy's stores will open on Thanksgiving for the first time in the company's history, and they will stay open through Black Friday evening. The company's chief stores officer noted that it was time for Macy's to join "the fun and excitement" of the Black Friday shopping spree. The company already had a significant presence on Thanksgiving, however. Macy's has sponsored the Macy's Thanksgiving Day Parade in New York since the 1920s. Last year, Macy's posted U.S. sales of about $27 billion in 2012, up 5% over 2011 sales.

6. Kohl's

Thanksgiving day opening: 8:00 p.m.

Most Kohl's stores will open Thursday evening at 8 p.m. and close again at midnight on Black Friday. This is the store's earliest ever Thanksgiving opening. The company is offering more than 500 online deals beginning on the Tuesday before Thanksgiving. Kohl's called the Black Friday shopping frenzy a "much-anticipated tradition enjoyed with family and friends" — sort of like Thanksgiving dinner. Kohl's posted U.S. sales of more than $19 billion in 2012, up 2.5% from 2011.

7. J.C. Penney

Thanksgiving day opening: 8:00 p.m.

J.C. Penney is opening its retail stores a full 10 hours earlier than it opened its stores last year. J.C. Penney posted U.S. sales of about $13 billion in 2012, a drop of nearly 25% from the prior year, and this year's sales are going to be much worse, judging from its performance so far. J.C. Penney cannot afford to stay closed if competitors like Kohl's and Macy's are open. Whether the extended hours will help the struggling retailer at all is questionable. But J.C. Penney already held what it called its "biggest sale ever" in early November. Whether the company chose to launch the sale to clear out old products before Black Friday, or to raise desperately needed cash, has not been disclosed.

8. The Gap

Thanksgiving day opening: 9:00 a.m. at Old Navy stores

The Gap plans to open more than 900 Old Navy stores from 9 a.m. to 4 p.m. on Thanksgiving Day. Additionally, about 700 stores will reopen at 7 p.m. that day and remain open until the end of Black Friday. In order to generate excitement, and compete for consumer dollars during a cutthroat holiday season, Old Navy plans to give away $1 million to one shopper during its Black Friday event. The company has not posted extended Thanksgiving hours for Gap or Banana Republic stores. Gap posted $12 billion in U.S. sales last year, up 5% from 2011.

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